Conventionally, in X-ray computed tomography apparatuses (hereinafter, also referred to as X-ray CT apparatuses), there is a case in which the dosage of X-rays (hereinafter, also referred to as “X-ray dosage”), which are radiated from an X-ray tube, varies at each time due to a factor such as a ripple of an X-ray tube voltage or a change with time. In this case, the dosage of the X-rays that pass through a subject and are detected by an X-ray detector also varies, resulting in a problem that a tomogram cannot exactly be reconstructed.
In the X-ray CT apparatus, in order to prevent this problem, a correction detector (reference detector) is disposed at a position where X-rays pass through only a region of air, or only a wedge filter, without passing through the subject, and the output of the X-ray detector is corrected based on the output of the correction detector. Since the output of the conventional X-ray detector is an integration-type output, it should suffice if the correction detector observes the total amount (the detector output relative to the total amount) in the entire energy range of X-rays. The integration-type output means an output indicative of a result of integration in the entire energy band of a product between an X-ray energy value and a detector sensitivity at this energy value.
On the other hand, there is known a photon-counting-type X-ray CT apparatus as an X-ray CT apparatus of a type different from the integration type. In the photon-counting-type X-ray CT apparatus, the main object is to acquire a tomogram at each X-ray energy or at each X-ray energy band (energy bin).
In this type of X-ray CT apparatus, there is known a method in which energy information of X-rays is specified by using a second X-ray detector of a photon-counting type. In this method, for example, when imaging is performed by switching an X-ray tube voltage on a view-by-view basis, such as when dual-energy imaging is performed, the energy information of X-rays, which are radiated from the X-ray tube, can be specified by the second X-ray detector. However, the energy information of X-rays, in this context, is information by which the X-ray dosage of a view can be acquired by using, e.g., an average value of a specific energy range, but this information is not information by which the energy spectrum of X-rays, which are radiated from the X-ray tube, can be estimated or corrected.
However, in the photon-counting-type X-ray CT apparatus, when the X-ray energy spectrum varies, although the X-ray dosage (the total or average value of the energy in the entire or specific range) can be observed by using a photon-counting-type X-ray detector, the variation of the X-ray dosage at each energy or at each energy band cannot be corrected. Thus, in the photon-counting-type X-ray CT apparatus, there is a problem that a tomogram at each energy or at each X-ray energy band cannot be reconstructed.
As described above, in the photon-counting-type X-ray CT apparatus, there is the problem that, when the energy spectrum of X-rays radiated from the X-ray tube varies, a tomogram at each energy or at each X-ray energy band cannot be reconstructed.
It is an object to provide an X-ray computed tomography apparatus which can correct a variation of the energy spectrum of X-rays radiated from an X-ray tube, and can exactly reconstruct a tomogram at each energy or at each energy band.